Red shift of the excited state due to a non-degenerate biexciton in self-organized quantum dots

When the n=1 (ground) state of quantum dots is resonantly pumped, a "non-degenerate" biexciton composed of an n=1 and n=2 (excited) states-pair may be observed with pump-probe spectroscopy. We observed this resonance and measured 15-meV binding energy in In
_0.4
Ga
_0.6
As self-organized quantum dots, consistent with theoretical calculations